Optical communication systems use pulses of light to transmit data. These optical communications systems use a number of components, such as lasers, the optical fibers, electroabsorption modulators (EAM's), semiconductor optical amplifiers (SOA's), and variable optical attenuators (VOA's), which encode signals in the pulses of light, transmit the pulses and detect the optical signals. Optical monitors are an important element of integrated optical component devices and systems. Optical monitors typically produce a current, or voltage, proportional to the optical energy incident on them. This electrical signal may be used to provide both performance monitoring and an input signal for performance optimization circuitry. Performance monitoring and performance optimization are desirable to ensure reliable operation of optical communication systems.
Desirably, an optical monitor should not produce large currents or use excessive amounts of electrical power, nor should the optical monitor create undue loss of the optical signal being monitored through scattering or absorption.
Currently in the telecommunications industry optical monitoring is accomplished using a fiber fused coupler that bleeds off about 5% of the optical signal and sends it to a separate optical detector. These couplers are larger and more expensive than may be desirable and also require fiber splicing in the system, which may lead to coupling losses. Additionally, some couplers, or other components, may be damaged during the delicate fiber splicing operations.
When deploying an optical transmitter into an optical communication network, it is also desirable to have algorithms that continually maintain and optimize the optical performance. One form of optical transmitter whose operation may be desirably improved by optical monitoring is an EAM. A valuable control algorithm for an EAM is to maintain a constant optical absorption across the modulator for the modulated signal. Maintaining a constant optical absorption across the modulator may ensure that the device is operating under the desired bias conditions. The ratio of this optical absorption to the transmitted signal is known as the extinction ratio of the modulator. Maintaining a constant extinction ratio may allow consistent optical performance, even with variations of wavelength and age-induced changes or environmentally-induced changes, such as temperature changes, in the performance of components of the optical communications system, including the EAM. Consistent optical performance for various input wavelengths may also be achieved.